This invention relates generally to integrated circuits and, more particularly, to nonvolatile memory arrays that use magnetic memory elements.
The semiconductor industry continuously strives to reduce the size and cost of memory, increase the speed for accessing memory, and improve the reliability of memory. One particular problem confronting the semiconductor industry is that of reducing the size of the memory cell in a Random Access Memory (RAM).
During the 1950s and 1960s, magnetic core memories were the predominant storage technology for the working memory of computing systems. The magnetic core memory technology was costly, and as the need for more fast on-line storage grew, there was no economical path to minimize the technology. At this time, it was proposed to supplant these devices with high density arrays of magnetic devices.
In the 1970s, magnetic core memories were replaced with integrated circuits, including static random access memory (SRAM) and including dynamic random ccess memory (DRAM) that is be periodically refreshed at frequent intervals. Non-volatile memories have been developed to address the problem of data volatility. For example, non-volatile memories include Electrically Erasable Programmable Read Only Memory (EEPROM) such as FLASH memory. Because of the rapid advancement in semiconductor density coupled with the advent of the DRAM cell, magnetic storage technology was not used for high-speed on-line memory, but rather was left to be used for low-cost, high-density memory in the form of various disk drive technologies.
It has been proposed to use magnetic storage such as Magnetic Random Access Memory (MRAM) technology as a replacement or supplement to the DRAM. In the MRAM structures that are being proposed, the capacitor storage element of the DRAM cell is replaced by a magnetic element. The magnetic element has a magnetic moment and is characterized by a predominant or easy axis of magnetization. In the absence of an external magnetic field, the magnetic moment is oriented along the easy axis of magnetization in one of two stable states. In magnetoresistance technology, one of the stable states for the magnetic moment of the magnetic element is a high resistance state and the other of the stable states is a low resistance state.
It has been proposed to construct a cross point array which would have a significant density advantage. In this device, the magnetic storage area, i.e. magnetic element, is located in the vertical space between the two wiring planes, i.e. the bit line and word line planes, arranged in an orthogonal pattern. Information is stored by the vector sum of the magnetic fields generated by an energized bit line and word line. These magnetic fields are perpendicular to each other. Assuming the currents are equal and produce a magnetic field having a unit value (1) strength, the resulting magnetic field is equal to the vector sum of the two fields. From the mathematical viewpoint, the resulting magnetic field is equal to 1.414 times the strength of the field generated by the current in one of the lines or conductors, and the resulting magnetic field has a line of force at 45 degrees to each line. If the direction of current flow in these conductors are reversed, then the line of force is at 180 degrees from the first case. As such, the easy axis of magnetization of the magnetic element intersects the point at which the orthogonal bit and word lines cross, and extends at an approximately 45 degree angle to each line.
Although it was assumed that the field in the storage area generated by both the current in the word and bit line are equal, the bit line is closer than the word line to the storage device in the structure of a simple magnetic tunnel junction (MTJ) device. Therefore a slightly higher field is generated by the bit line if the current in the word and bit line are equal.
The current in the selected bit line and in the selected word line generates a magnetic field of a magnitude equal to a unit value (1) at right angles to each cell it traverses. Half-select errors occur when the magnetic field is sufficient to write data to a magnetic storage element. The resulting magnetic field generated by the sum of the currents in the bit line and the word line must be sufficient to write the most difficult magnetic storage element. If the easiest element can be written by a field of less than approximately fifty percent of that needed to write the most difficult element, then the easiest element will be written by the field generated by the current flowing in one conductor, i.e. either through the bit line or the word line alone when another cell along the bit line or word line is being written. To prevent half-select errors in a situation in which the magnetic fields generated by the current in the bit lines and the word lines are equal, each storage element is fabricated so as not to be significantly less than 50 percent different from any other. In the situation in which the magnetic fields are not equal because, for example, the bit line is closer than the word line, there is less margin for error in the fabrication of the storage element. For example, if 70% of the motive force for writing a storage element is attributed to current in one of the lines, then the margin of error is less than 30%. Otherwise, the current in the line that provides 70% of the motive force produces a sufficient magnetic field to write to the cell unintentionally. As such, great care in photo-processing film deposition and composition must be achieved.
Therefore, there is a need in the art to provide a system and method that overcomes the half- write problems for magnetic memory elements.
The above mentioned problems are addressed by the present subject matter and will be understood by reading and studying the following specification. The present subject matter provides three terminal magnetic random access memory structures and methods. A word line, bit line and select line that traverse a given bit are energized to access the given bit. A significantly increased margin of safety or tolerance for the difference in magnetic susceptibility of the individual bit positions is achieved by energizing three lines rather than two lines. Thus, the probability of half-select errors is significantly diminished.
One aspect of the present subject matter is a memory cell. One embodiment of the memory cell includes a first conductor line, a second conductor line, a third conductor line, and a magnetic storage element. The magnetic storage element is operably positioned to be magnetically coupled to a first magnetic field produced by an energized first conductor line, to a second magnetic field produced by an energized second conductor line, and to a third magnetic field produced by an energized third conductor line. The magnetic storage element is adapted to be written by a vector sum of the first magnetic field, the second magnetic field, and the third magnetic field.
One aspect of the present subject matter is a method for writing to a magnetic storage device. According to one embodiment of this method, a first magnetic field vector, a second magnetic field vector and a third magnetic field vector are formed at the magnetic storage device. The magnetic storage device is written by a vector sum of the first magnetic field vector, the second magnetic field vector and the third magnetic field vector.
One aspect of the present subject matter provides a method for writing to a magnetic storage device. According to one embodiment of this method, a word line is energized to generate a first magnetic field vector through the magnetic storage device, a bit line is energized to generate a second magnetic field vector through the magnetic storage device, and a select line is energized to generate a third magnetic field vector through the magnetic storage device.
These and other aspects, embodiments, advantages, and features will become apparent from the following description of the invention and the referenced drawings.